This invention relates to an imaging array and system, and a method of operating the same, in which a large number of closely spaced imaging devices, preferably light emitting diodes (LEDs) optically coupled to a common lens system and electronically coupled into an organized driving system, are capable of high speed exposures of incremental areas of a photoreceptor. An image or images can thus be formed of a large number of small pixels, at resolutions in the order of three hundred lines per inch, on copy material of various types, and at speeds in the order of 100 to 300 ft./min.
The combination of such resolution and speed has been heretofore unattainable as a practical matter. Relatively sophisticated copy machines have been developed, using powder toner, and while a few of these have capability of printing on web material those units are essentially a variation of similar sheet fed copiers. They all have operating speeds in the eighty to ninety copy/minute range, and this speed is fixed. Their exposure and development systems will not tolerate variation in speed. Such prior art copiers, by their very nature, are also sensitive to characteristics of the copy material, e.g the sheet on which the copy is made. In general, those copiers have difficulty making good reproductions on certain coated papers, or on material of variable thickness as where blank labels are already adhered to the material.
Diode array exposure systems have been known, such as disclosed in U.S. Pat. Nos. 4,455,562, 4,596,995 and 4,780,731, however due to limitations in data handling capacity and LED optical output power, prior art arrays have been limited to relatively low speeds and to array lengths in the order of twelve inches (the top-to-bottom dimension of most text to be copied). Higher speeds are not attainable due to the manner in which data is transferred to the individual LEDs of an array, and in the power limitations which prevent light radiated from such prior art arrays from effectively forming an electrostatic image on a photoreceptor surface.
Furthermore, it is known that an array of large numbers of LEDs will exhibit differences in light intensity output among the individual LEDS. Such intensity variations can cause significant differences in the discharging of pixel areas on a charged photoreceptor surface (such as on a rotating drum). As speeds are increased, the resulting problems magnify. Thus, a number of ways have been suggested to compensate for the LED light output variations. One scheme requires that the array be scanned, one LED at a time, and the observed differences in light output from the individual LEDS is used to develop a look-up table. Such a table in turn is read to modify the on-time of the LEDs to produce a closer average light output from all LEDs in the array each time they are individually driven in response to a character generating input. Another scheme uses a light detector in the array to detect the outputs of the LEDs when they are driven, and to provide immediate feedback compensation to the driving circuit.
However, the clocking and scanning circuits used for driving the LED arrays are generally synchronized to motion of the photoreceptor, e.g. drum rotation. Such compensation schemes have not, heretofore, taken into account the fact that by lengthening the on-time of certain ones of the LEDs, the mid-point of LED on-time is varied. In other words each LED to be driven, in exposing desired ones of a row of pixels across the photoreceptor, will initiate exposures at the same instant, but exposures will stop at different times. Thus, centering of the LED ontime with respect to the desired center of the pixel area on the photoreceptor is disturbed by those compensation schemes.